Mobile IP is a mechanism for maintaining transparent network connectivity to and from a mobile node (MN), such as a mobile terminal or telephone, over an IP based network whilst the mobile node is roaming within or across network boundaries. Mobile IP enables a mobile node (or rather user) to be addressed by a fixed IP address (a “Home Address”) allocated by its home network, regardless of the network to which it is currently physically attached. The Home Address causes all traffic sent to the Home Address to be routed through the home network. Ongoing network connections to and from a mobile node can be maintained even as the mobile node is moving from one subnet to another. Mobile IP can be implemented using IP version 4 (IPv4) or IP version 6 (IPv6), although IPv6 is generally preferred as IPv4 has a number of limitations in a mobile environment. The IPv6 protocol is specified in RFC 2460, whilst Mobile IP using IPv6 is specified in IETF RFC 3775, ‘Mobility Support in IPv6’.
According to Mobile IPv6, a mobile node is always reachable via its Home Address. However, while away from its home IP subnet (Home Subnet), a mobile node is also associated with a Care-of Address that indicates the mobile node's current location. The mobile node can acquire its Care-of Address through a conventional IPv6 mechanism called “auto-configuration” within the visited IP subnet. The association of the mobile node's Home Address and the Care-of Address is known as a “Binding”. A router in the Home Subnet, known as the “Home Agent”, maintains a record of the current Binding of the mobile node. Mobile IPv6 requires functionality in the TCP/IP Protocol stack of a mobile node, as there must be an exchange of signalling messages between the mobile node and its Home Agent to enable the creation and maintenance of the binding between the mobile node's Home Address and its Care-of-Address.
Proxy Mobile IPv6 (PMIP) is a new standard currently being developed by the Internet Engineering Task Force (IETF) (see IETF Request for Comments 5213). PMIP is a network-controlled IP mobility protocol that is transparent to the mobile node and will be used for mobility management in the 3GPP Evolved Packet System. This approach to supporting mobility does not require the mobile node to be involved in the exchange of signalling messages with a Home Agent. A proxy mobility agent in the network performs the signalling with the Home Agent and performs the mobility management on behalf of the mobile node.
The architecture of a Proxy Mobile IPv6 communications network is shown in FIG. 1. The core functional entities in PMIP are the local mobility anchor (LMA) 2 and the mobile access gateway (MAG) 3. The local mobility anchor 2 is the Home Agent for the mobile node in Proxy Mobile IPv6. The local mobility anchor 2 is responsible for maintaining the mobile node's 1 reachability state and is the anchor point for the mobile node's home network prefix. The mobile access gateway 3 is the entity that performs the mobility management on behalf of a mobile node 1 and it resides on the access link where the mobile node 1 is attached. The home network prefix is assigned to the link between the mobile node 1 and the mobile access gateway 3. The mobile access gateway 3 is responsible for detecting the mobile node's movements to and from the access link and for initiating binding registrations to the mobile node's local mobility anchor 2.
A bidirectional tunnel between the local mobility anchor 2 and the mobile access gateway 3 ensures that the mobile node 1 can communicate as if it was physically located in its home network. The local mobility anchor 2 and mobile access gateway 3 perform the necessary encapsulation and decapsulation on behalf of the mobile node 1. The local mobility anchor 2 receives any packets that are sent by any correspondent node to the mobile node and forwards these received packets to the mobile access gateway 3 through the bi-directional tunnel. The mobile access gateway 3 removes the outer header from the packet and forwards the packet on the access link to the mobile node 1. Any packet that the mobile node 1 sends to any correspondent node will be received by the mobile access gateway 3 and will be sent to its local mobility anchor 2 through the bi-directional tunnel. The local mobility anchor 2 removes the outer header and routes the packet to the correspondent node. In the Evolved Packet System, the Packet Data Node (PDN) gateway takes on the role of the local mobility anchor, and serving gateways act as the mobile access gateway.
A multi-homed mobile node is a mobile node that simultaneously attaches to the Proxy Mobile IPv6 domain through multiple interfaces. Each of the attached interfaces of a multi-homed mobile node is assigned one or more unique home network prefixes. However, if a multi-homed mobile node performs a handoff by moving its address configuration from one interface to the other, the local mobility anchor will assign the same home network prefix(es) to the new interface, as are assigned to the old interface. As such, PMIP requires the old interface to be disabled after the handover in order to avoid a conflict between the existing IP configuration on the old interface and the newly established, identical IP configuration on the new interface. Therefore traffic can only be handed over on a per-interface basis. This lack of support for the hand-over of individual traffic flows between the interfaces of a multi-homed mobile node in Proxy Mobile IPv6 is a crucial shortfall given the desire of many operators to support this feature for simultaneous multi-access.